Optical signals do not diffract as much as radio frequency (RF) signals. This makes them useful for a variety of ranging and imaging radar-like applications using relatively small apertures. One common light detection and ranging (lidar) method is a phase-shift method where an optical signal is modulated by a RF frequency, and the phase of the RF frequency of the return optical signal (the return optical signal is the signal that bounces off an object to be measured and returns to the transceiver) is measured thus giving information about the distance to the object. The phase can be monitored over time (a phase change with respect to time is equivalent to an RF frequency shift) to determine the speed of the object with respect to the transceiver. Avalanche photodiodes (APDs) are sometimes used as the optical detector since they have a large internal gain making them sensitive to the small levels of return light typically encountered, especially when the transceiver and object are far apart. The electrical signal from the APD can be mixed with an RF local oscillator in a mixer to translate the received signal frequency down to a level where signal processing can more easily be performed.
It would be advantageous in terms of sensitivity to use a single photon sensitive detector (SPD). However, such detectors are not generally linear with respect to the input optical signal, thus a traditional mixer is not necessarily a preferred component for processing the SPD output. APD's can be operated in a SPD mode (the Geiger mode) where they are sometimes used in lidar to measure the time-of-flight of a pulse from a transmitter to the object and back again because of their very high sensitivity to small levels (single photons) of reflected light. The time-of-flight can be translated into distance to the object since the speed of light is constant. The optical pulse repetition rate in a time-of-flight scheme is typically quite low, making it difficult to perform velocity measurements or fast measurements suitable for imaging when using SPDs.
When using a phase shift detection method for the purposes of measuring velocity or for measuring distance to a high resolution, it is advantageous if the RF frequency of the optical modulation is high, for instance in the GHz range, since a higher frequency makes a small distance displacement cause a relatively large change in phase. Unfortunately the fairly high bandwidth APDs required in such a situation will typically have a poorer optical sensitivity specification. The RF output signal from an APD operating in linear mode can be mixed with a RF local oscillator to bring the received signal frequency down to a level where signal processing can more easily be performed. As an example, if the RF modulation signal is fm=3 GHz then the expected Doppler frequency shift is δf˜2·fm·ν/3×108=20·ν Hz where ν is the relative speed in m/s between the transceiver and the object and 3×108 m/s is the speed of light. For an object moving at 10 m/s the corresponding frequency shift is 200 Hz. The frequency shift is proportional to fm, which makes the use of high modulation frequencies attractive. The ˜3 GHz mean frequency can be mixed with an RF local oscillator of, say 3 GHz+100 kHz in a mixer, thus creating a manageable 100 kHz intermediate frequency (IF) signal when ν=0. By monitoring the output frequency of the mixer, the magnitude and direction of the relative object velocity can be measured. The phase of the output frequency of the mixer can be monitored in order to measure the range to the object, with a ranging ambiguity of modulo 2π, which in this case leads to an unambiguous range dunambiguous=c/(2·fm) of 5 cm. There are various ways of extending the ranging ambiguity that are known in the art, such as the use of multiple modulation frequencies (U.S. Pat. No. 5,589,928). The highest modulation frequency typically limits the ranging resolution.
FIG. 1 shows a possible implementation of a phase shift ranging and/or velocity measurement system that is consistent with the aforementioned discussion and prior art. A continuous wave (CW) laser 100 is modulated in a modulator 102 by a high frequency sinusoidal wave generated in an oscillator 101. A sinusoidal wave is imparted to the amplitude of the laser light via the modulator and sent to an object through an optical antenna 104. A small portion of the transmitted light is reflected and returns to the transceiver 100 through a receive optical antenna 108. The return light is sent to an APD 110 operating in linear mode which converts the received optical signal into an electrical signal. The oscillator 101 is frequency shifted by a fixed amount in a frequency shifter 106. The output of the frequency shifter is mixed with the analog APD output in a mixer 114 and the resulting return signal is digitized in a signal analog-to-digital converter (ADC) 116 the output of which is sent to the processor 122 for processing. A reference signal obtained by mixing the frequency shifted light from the frequency shifter and the oscillator output signal in a reference mixer 118 is digitized using a reference ADC 120 and the digitized reference signal is sent to the processor. The frequency difference between the digitized reference signal and the digitized return signal contains the relative velocity information and the phase difference contains range information. This example is illustrative and the actual components that are employed can be modified, as can the type of modulation and other modifications as is known in the art. The system allows the phase of a return optical signal to be monitored as a function of time and used to determine either range and/or velocity information of the object.
It would be advantageous in terms of sensitivity to use a single photon sensitive detector (SPD). However, such detectors are not generally linear with respect to the input optical signal so they are not well suited for the phase shift measurement of FIG. 1. For instance, APDs operating as SPDs often incorporate digitizers for operating as digital output devices, typically generating a binary one level if one or more photons are detected in a given time frame or generating a binary zero level if no photons are detected. SPDs can have other linearity issues. For instance, an SPD can experience a time period directly after a photon is detected where the detector has some performance degradation such as a relatively high probability of detecting a photon even if none are present. Some SPDs effectively disarm the detector during this time period, which creates a detector dead time after a detection event where the detector is unable to detect additional photons. We will refer to the dead time more generally as a period of reduced performance after detection, since the detector can be disarmed during this period if needed.
A photon counting ladar system that does measure phase is described in U.S. Pat. No. 7,675,610 B2. This system modulates a laser with a chirped transmit waveform and gates the detector with the same waveform. The transmit modulation and gate modulation are thus the same signal (both frequencies change with time which is known as a chirp). By modulating the detector gain and low-pass filtering the resulting binary detector output this ladar system essentially mixes the transmit and receive frequencies. The received signal after mixing and filtering has a frequency that depends on the range to the object. The method requires chirped signal generation and its range resolution is limited by the chirp parameter. Range resolution is typically limited by ˜c/2 B where B is the bandwidth of the chirp which for a 200 MHz chirp limits the range resolution to ˜75 cm even if the APD detection time windows caused by the gates are sub-ns. It would be advantageous not to have to generate such chirped signals and to maintain a range resolution limited by the short sub-ns detection time windows possible with gated APDs rather than being limited by the chirp bandwidth.
Additionally U.S. Pat. No. 7,675,610 B2 does not mitigate the impact of detector dead time since the optical signal and gated photon detection window are overlapped for many consecutive gates. This limits the linearity of the system and reduces the number of photons that can be detected when the received signal is high. For instance, if the mean gating frequency is ˜2 GHz and the low pass filtered signal (whose frequency depends on the range to the object) is 25 MHz, then the gates and optical pulses will move from being very well overlapped (high detection efficiency) to very poorly overlapped (near zero detection efficiency) and back again over the course of 80 gates. If the detector time-gates create a high detection efficiency region with a 20% duty cycle (125 ps time windows of the ˜500 ps gate repetition rate having high detection efficiency) then for about 16 consecutive gates (or 8 ns) the optical signal will be well overlapped with the gates (thus having high detection efficiency) while for the remaining 64 gates they are poorly overlapped. However, if the detector dead time is 8 ns then even if the optical signal is very large (say many 10's of photons per pulse) the detector count rate will saturate at about one counts during each 40 ns cycle. Such a design limits the linearity of the system. It would be more optimal if the counts were more evenly spaced throughout the detection process, ideally spaced by at least the detector dead time, so that the process could be more linear over a wider dynamic range of the input photon flux. If this could be done, then the equivalent maximum count rate would be ˜40 ns/8 ns=5 counts per 40 ns.
Prior art has used phase shift detection in the RF domain (U.S. Pat. No. 5,455,588) and optical domain (U.S. Pat. No. 6,697,148). Single photon detectors have been used in lidar including with the use of modulated pulse streams for time-correlated single photon counting (McCarthy et al, Applied Optics 48 pp 6241-6251, 2009), but these systems used a free running (ungated) detector. High speed gated single photon detectors have been previously used in lidar (Min Ren et al, “Laser ranging at 1550 nm 1-GHz sine-wave gated InGaAs/InP APD single-photon detector,” Opt. Express 19, 13497-13502, 2011), but the gate and optical pulse repetition frequencies were unsynchronized and the optical pulse rate was much smaller than the detector gate rate. This ‘free running’ gated mode is used as a proxy for an un-gated APD since ungated APDs are difficult to realize in the 1550 nm band. The ungated or the gated free running modes do not typically measure phase but rather the time of flight of the laser pulse to and from the object. This does not typically make for fast measurement times nor is it conducive to measuring velocity.
Applying a time-gated voltage across an APD, and also some other types of SPDs like gated superconducting nanowire based SPDs (Mohsen K. Akhlaghi and A. Hamed Majedi, “Gated mode superconducting nanowire single photon detectors,” Opt. Express 20, 1608-1616 (2012)) causes an unwanted signal to feed-through the device that makes detecting the small breakdown signals which indicate a photon detection more difficult. Detecting small breakdowns is beneficial since large charge flows cause greater trapped carriers which in turn causes an unwanted afterpulse effect where the device can break-down upon receiving a gate pulse even when no photons are present. This afterpulse effect can be controlled by waiting a suitably long time between gates to allow the carriers to disperse. However waiting a long time between gates slows down operation and in any event at high gating rates it is not always technically feasible to turn off the gate signal after a detection event. A method to account for afterpulsing when gating at high speeds is to continually gate the detector but ignore breakdowns recorded for a given time period after a photon is detected. This time period can be considered equivalent to the detector dead time.
Recent work in the field of single photon counting has suggested that the use of either a sine wave gate or the use of differential subtraction can allow small breakdowns to be detected using suitable analog processing to reduce the feed-through signal. A method that subtracts a reference signal from the SPD output has been proposed [patent application U.S. 2011/0127415 A1 by G. S. Kanter]. The reference signal subtraction method is relatively flexible, allowing for the gate frequency to be easily reconfigured.
What is needed is a system for measuring low photon levels that can identify the phase of the incoming signal. The system can be used for fast, precise ranging and velocity measurement and can make use of very sensitive single photon detectors. The system should be capable of high resolution ranging, but can also ideally allow for unambiguous ranging over long distances. The ranging resolution can exploit the short detection time windows possible by time-gating a single photon detector. A chirped signal should not be required. Issues stemming from the detector dead time or other linearity problems in SPDs should be mitigated to maintain the highest possible dynamic range and fastest signal acquisition. Practical components should be employed to construct the device.